1. Field of the Invention
This invention relates to a method for obtaining three-dimensional information of a sample and an apparatus for carrying out the method. This invention particularly relates to a method and apparatus for obtaining three-dimensional information of a sample, wherein a sample is scanned with a laser beam, and three-dimensional information representing the form and/or structure of the sample, such as information representing a tomographic image or a three-dimensional image of the sample, or three-dimensional information representing constituents and/or functions of the sample is thereby obtained such that the sample may not be destroyed. This invention also relates to a method and apparatus for obtaining three-dimensional information of a sample, such as information representing a tomographic image or a three-dimensional image of the sample, wherein a sample is helically scanned with an electromagnetic wave, and three-dimensional information of the sample is thereby obtained such that the sample may not be destroyed.
2. Description of the Prior Art
It has heretofore been desired that three-dimensional information representing the form and/or structure of a sample, i.e. three-dimensional information representing the external form and/or external structure of the sample or three-dimensional information representing the internal form and/or internal structure of the sample, can be obtained such that the sample may not be destroyed. In particular, in the biological and medical fields, the sample is a living body, and therefore there is a strong demand for a technique for obtaining three-dimensional information representing the form and/or structure of the sample such that the sample may not be destroyed.
As techniques for satisfying such a demand, X-ray computed tomography (X-ray CT) and magnetic resonance imaging (MRI) enable tomographic images of samples to be obtained. However, in the strict sense, the tomographic image is two-dimensional information. Therefore, research has still been continued in order to obtain three-dimensional information of samples.
As a technique for obtaining three-dimensional information of a sample, a multilayer X-ray CT technique has been proposed. With the multilayer X-ray CT technique, a single X-ray beam is irradiated to a sample and rotated 360.degree. along a circumference of a circle, which lies in a plane normal to an imaginary axis (hereinafter referred to as the body axis) of the sample and which has its center at the body axis. A signal representing an image of a tomographic layer of the sample is calculated from the amount of the X-ray beam, which has passed through the sample. The X-ray beam is then moved along the body axis and rotated in the same manner, and a signal representing an image of a tomographic layer of the sample is calculated in the same manner. A signal representing an image of a tomographic layer located between the two tomographic layers is interpolated from the signals representing the two tomographic layers, and three-dimensional information of the sample is thereby obtained.
The multilayer X-ray CT technique has advanced to a helical scanning X-ray CT technique, wherein an X-ray beam is moved helically around a body axis of a sample, and an image signal which is serial in the body axis direction is thereby obtained.
However, with the aforesaid X-ray CT technique and the MRI technique, three-dimensional information of a sample cannot be obtained directly. Specifically, with the aforesaid X-ray CT technique, the sample is exposed to a spot of the X-ray beam, and therefore the thus detected image is merely the line segment-like information obtained through displacement of the spot of the X-ray beam. Therefore, a helical scanning X-ray CT technique utilizing a conical beam, wherein a surface-like X-ray beam is irradiated to a sample and an image signal representing a surface-like region is thereby obtained, has been proposed in, for example, "Three-dimensional Helical Scanning CT Utilizing Conical Beam Projection" in Symposium of The Institute of Electronics and Communication Engineers of Japan, D-II, Vol. J74-D-II, No. 8, August 1991. However, no means has been developed for detecting a surface-like X-ray image, i.e. a two-dimensional X-ray image, which is obtained by the helical scanning X-ray CT technique utilizing a conical beam. Therefore, the proposed technique remains at the stage of proposal and has not yet been put into practice.
A spectroscopic analysis method is useful for identification of a substance, and therefore an attempt has been made to obtain an image, which represents an internal form and/or an internal structure of a sample, by utilizing light. Also, in the field of optics, a means for detecting light as a two-dimensional image has already been developed. For example, a charge coupled device (CCD) camera can detect light as a two-dimensional image.
Accordingly, as a method for obtaining three-dimensional information of a sample by utilizing light, a method utilizing an optical CT microscope has been known.
With the optical CT microscope, a surface-like laser beam is irradiated from an oblique direction to a sample, and the laser beam source, which produces the laser beam, is moved along a circumference of a bottom surface of a cone having its vertex at the sample. In this manner, an image of the laser beam, which has passed through the sample from different angles, is recorded by a CCD camera. An image reconstruction process utilizing the CT technique is carried out on the recorded image of the laser beam, and three-dimensional information of the sample is thereby obtained. (Such a technique is described in "Optical CT Microscope and Three-dimensional Observation," Optical Technology Contact, Vol. 28, No. 11, 1990.)
However, with the aforesaid apparatus for obtaining three-dimensional information of a sample, the direction, from which the laser beam is irradiated to the sample, is limited. Therefore, the signal representing the cross-sectional image along the directions of the passage of the laser beam cannot be obtained sufficiently, and the accuracy of the cross-sectional image cannot be kept high.
Also, during the image reconstruction process, an approximation method (Born approximation), in which the sample is limited to an approximately transparent body, is utilized. Therefore, the above-described method for obtaining three-dimensional information of a sample cannot be applied in cases where the sample contains a light scattering medium as in the cases of a living body such that scattered light is radiated out of the sample together with light, which has passed through the sample.
It has also been desired that three-dimensional information representing constituents and/or functions of a sample can be obtained such that the sample may not be destroyed. In particular, in the biological and medical fields, the sample is a living body, and therefore there is a strong demand for a technique for obtaining three-dimensional information representing constituents and/or functions of the sample such that the sample may not be destroyed.
As a method for obtaining three-dimensional information representing constituents and/or functions of a sample, a positron CT (PET) technique is known. Though the positron CT technique enables constituent information to be obtained to some extent, it has the drawbacks in that a large-sized facility, i.e. a cyclotron, must be used, and there is the risk that the sample is exposed to radiation.
A spectroscopic analysis method is useful for identification of a substance, and therefore an attempt has been made to obtain an image, which represents constituent information of a sample, by utilizing light.
For example, research has heretofore been carried out on laser beam scanning microscopes as devices utilizing a laser beam. The laser beam scanning microscopes are roughly classified into a reflection type and a transmission type. The reflection type of laser beam scanning microscope is useful for obtaining three-dimensional information representing the form and/or structure of a sample, e.g. a tomographic layer of a fluorescent sample or unevenness of a reflective sample. However, with the reflection type of laser beam scanning microscope, information representing constituents and/or functions of the sample cannot be obtained.
With the transmission type of laser beam scanning microscope, it is intended to obtain information on light absorption by constituents of a sample from the laser beam, which has passed through the sample, and thereby to obtain information representing constituents of the sample. However, the information on light absorption is obtained as a result of integration of the constituent information corresponding to the region of the sample through which the laser beam has passed. Therefore, three-dimensional information representing constituents of the sample cannot be obtained.
As a technique for solving the problems described above, the method for obtaining three-dimensional information of a sample by utilizing an optical CT microscope has been known. However, as described above, this conventional technique has various drawbacks.